1. Field of the Invention
This invention relates to a thermosensitive recording material, and more particularly to a thermosensitive recording material comprising a thermosensitive coloring layer formed on a support, and an overcoat layer formed on the thermosensitive coloring layer, which overcoat layer comprises a resin crosslinked by a glycidylamine compound. The overcoat layer can be formed by coating a coating liquid comprising the resin and the glycidyamine compound.
2. Discussion of Background
Conventionally, there is known a thermosensitive recording material, in which a thermosensitive coloring layer comprising as the main component a thermosensitive coloring composition is provided on a support such as a sheet of paper or synthetic paper, or a plastic film. In such a recording material colored images ar obtained by application of heat to the recording material using a thermal head, a thermal pen, laser beam, a stroboscopic lamp, or the like.
This type of thermosensitive recording material has wide-scale utilization, not only as a recording material for copying books and documents, but also as a recording material for use with printers for electronic calculators, facsimile machines, ticket vendors, and label-recorders because of the following advantages over other conventional recording materials:
(1) images can be speedily recorded by using a comparatively simple device without complicated steps such as development and image fixing;
(2) images can be recorded without generating noise and causing environmental pollution; and
(3) the manufacturing cost is low.
The thermosensitive coloring composition for use in this type of thermosensitive recording material usually comprises a coloring agent and a color developer capable of inducing color formation in the coloring agent upon application of heat thereto. Conventionally, as such coloring agents, colorless or light-colored leuco dyes having, for example, lactone, lactam, or spiropyran rings, are employed; and as such color developers, organic acids and phenolic materials are employed. The thermosensitive recording material in which the aforementioned leuco dye and color developer are used in combination is widely used, since the color tone of the produced images is clear, the whiteness degree of the background of the thermosensitive recording material is high, and the produced images have excellent weathering resistance.
The above-mentioned thermosensitive recording material, however, has the shortcoming that recorded images become easily discolored or fade away when the recorded images come into contact with oils and plasticizers such as dioctyl adipate and dioctyl phthalate contained in a plastic film, and that the background area is colored. As a solution to the above problems, it has been proposed that a protective layer be provided on the thermosensitive coloring layer. Japanese Laid-Open Patent Application No. 56-146794 discloses a thermosensitive recording material comprising a protective layer which comprises water-soluble resins such as polyvinyl alcohol and starch, crosslinked by crosslinking agents such as glyoxal and epoxy compounds, and auxiliary agents for matching improvement such as a wax and a filler of, for instance, calcium carbonate or silica. Such a thermosensitive recording material can attain a certain degree of oil resistance and plasticizer resistance. However, the problems related to oil resistance and plasticizer resistance of the thermosensitive recording material are in fact not completely solved.
It is also proposed that two protective layers be laminated to improve the oil resistance of the thermosensitive recording material, as disclosed by Japanese Laid-Open Patent Applications No. 59-45191 and No. 59-67081. However, such thermosensitive recording materials have the shortcoming that the color formation sensitivity is poor.
Another problem of the protective layer is water resistance: The protective layer is peeled off the thermosensitive coloring layer when it comes into contact with water. Japanese Laid-Open Patent Application No. 60-192683 discloses that the problem of water resistance can be solved to some extent by containing melamin and an acid catalyst in the protective layer. In this case, however, the thermosensitive recording material has the shortcoming that the pot life of a coating liquid for the protective layer is short.